Color television tube target structure



May 8, 1956 E. o. LAWRENCE COLOR TELEVISION TUBE TARGET STRUCTURE FiledDec. 22, 1953 4 Sheets-Sheet INVENTOR. fin/:'57 waff/vc! #fray/Vix! May8, 1956 E. o. LAWRENCE 2,745,035

COLOR TELEVISION TUBE TARGET STRUCTURE Filed DBC. 22, 1955 4Sheets-Sheet 2 I N V EN TOR. [PA/[5r O, nuff/vc! WSW May 8, 1956 FiledDec. 22,

E. O. LAWRENCE 2,745,035

COLOR TELEVISION TUBE TARGET STRUCTURE 1953 4 Sheets-Sheet 3 Fla. 6

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E. O. LAWRENCE COLOR TELEVISION TUBE TARGET STRUCTURE May s, 1956 FiledDec. 22, 1953 nited States Patent CoLoR TELEvisIoN TUBE TARGET STRUCTUREErnest 0. Lawrence, Berkeley, Calif., assigner to Chromatic TelevisionLaboratories, Inc., New York, N. Y., a corporation of CaliforniaApplication December 22, 1953, Serial No. 399,754 Claims. (Cl. 315-14)This invention relates to target structures employed in cathode-raytubes designed for the display of television images in substantiallynatural color. t constitutes a continuation-in-part of copending UnitedStates. patent application, Serial No. 265,366, led by the same inventoron January 8, 1952, for a Display Surface For Color Television Tubes,now U. S. Patent No. 2,669,675, granted February 16, 1954.

The invention relates to tubes of the type in which a plurality ofdierent phosphors which are emissive of light, on electron impact, ofdiierent component colors additive to-produce white light, aredistributed in a repetitive pattern which covers substantially theentire area of a display screen which forms one element of the targetstructure, The sub-areas of the screen occupied by the individualphosphors in tubes of this type are in at least one dimension of smallersize than the elemental areas or picture points of the images to bereproduced by the system and the display of individual colors iscontrolled by conlining the beam of cathode rays which traces the imageto the particular phosphor or phosphors emissive of the color desired.

Since in the ordinary process of scanning a display surface the beam iscaused to traverse successively substantially all portions of thedisplay screen, means must be provided in tubes of this type forrestricting that portion of the beam which actually impacts the screento an area which is no larger than the sub-area within each pictureelement which is emissive of the desired colored light. Various tubeshave been devised in which this restriction is accomplished by a maskwhich intercepts that portion of the beam which would otherwise strikethe undesired phosphor or phosphors. ln the case of tricolor imagereproduction (which is preferred as giving the best compromise betweencolor iidelity and excessive bandwidth and apparatus complication) theuse of such a mask presupposes that at least two-thirds of the energyavailable within the beam must be wasted.

, ln the type of tube to which this invention specifically pertains therestriction of the beam to a size which will limit it to an individualphosphor sub-area is accomplished by means of a multiplicity of electronlenses which converge the beam to a size smaller than that of theapertures through which it falls. It is the structure used so toconverge the beam that forms the second element of the target. This maybe a perforated screen or one or more grids comprised of linearconductors; however it may be constructed, whether of a singleperforated plate, a single array of approximately parallel wires ornarrow strips or tapes or a plurality of such sets of conductors, itsfunction is the same, and for convenience it will generally be referredto hereinafter as the lensgrid or simply the grid."

The conductors of the lens-grid may occupy a very small proportion ofits over-all area and the proportion of the beam intercepted by thestructure is thus reduced from a minimum of two-thirds to something intherneighborhood of fteen percent or even less. Arrangements of2,745,035 Patented May 8, 1956 One of the most satisfactory forms ofelectron lens l for accomplishing such post-deflection focusing, as ismentioned above, utilizes the display screen itself (which is madeconducting) as one element of the multiplicity of electron lenses.Preferably conductivity is provided by depositing a thin lilm of metal,preferably aluminum, on the surface of the phosphor layer which coversthe screen so that the metal film is faced toward the electron beamsource. The grid structure, already described generally, is biased by avoltage which is substantially the same as that used to accelerate thebeam as it issues from an electron gun (or guns) of substantiallyconventional type. The conducting lilm on the screen is made positivewith respect to the grid, and by properly adjusting the ratio of thevoltage applied to accelerate the beam to that between grid and screenal greater or less degree of convergence of the beam, after passing thegrid, may be obtained. In addition to the advantage of simplicityoffered by such a lens structure there is the Y additional advantage inthe fact that a relatively low voltthus broadly indicated. Among theobjects of the invention are to provide a target structure which willdisplay the saine color over all parts of the screen under givenconditions of color control applied to the tube, and thus give equaldelity of color reproduction throughout the picture field; to providemeans and methods for so positioning the phosphor areas with relation tothe grid as to accomplish such color fidelity; to provide means andmethods of the character described which are applicable to tubes of manytypes, including both those which use a plurality of electron guns andachieve their color control by virtue of the angle of incidence of theVarious beams at the grid and those wherein the color control effectedby means of micro-deflection at the grid; to

provide display screen conformations and methods of production of tubeswhich fail to meet the tolerances actually imposed.

Broadly consideredthe invention comprises a target structure including asubstantially plane base on which phosphors emissive of a pluralityof'component colors additive to produce white are deposited in arepetitive pattern of groups, each of which includes all of thephosphors employed, the dimensions of each group being, in one directionatleast, of the order of magnitude of one elemental area of the pictureto be' reproduced. Means are provided for rendering the screenconducting and for applying an acceleration potential vthereto relativeto a grid, which is mounted in a plane substantially parallel to thescreen and is provided with apertures corresponding in number to thephosphor groups. The spacu ings .0f the phosphor groups .are Soproportioned with ,respect to the spacings of the aperture centers thatthis ratio is greater than unity and is less than the ratio of thedistance between the center of scanning deflection vof the cathode-raybeam and the screen to the distance .between that center of deiiectionand the grid.

The nature of the invention will be better understood by reference tothe following detailed description of a n number of specific embodimentsthereof, taken in conjunction with the accompanying drawings wherein:

Fig. l is a schematic illustration of a cathode-ray tube of a typeembodying the instant inYention, ,Operating-circuits for this tube beingillustrated in block form;

Fig. 2 is an illustration of a portion ,of one type of display screen asused in the tube illustrated in Fig. 1, showing the pattern in which thephosphors are .disposed on the screen;

Fig. 3 is a similar view of a portion of a display screen f wherein thephosphors are disposed `in a linear or a strip pattern;

Fig. 4 comprises graphs illustrative of .the relationships,

Y in a tube using a single grid'of linear conductors, ofy

voltages used to accelerate the electron beam to the size of the spot onthe display screen produced by beam im.

pact;

Fig. 5 is a diagrammatic illustration showing an elec-` tron trajectorybetween a single grid and the display screen;

Fig. 6 is a series of graphs illustrating the displace,- ment of thefocal spot upon the screen, with respect to the perpendicular Idroppedfrom the centers of the apertures in the grid, with varying anglesV of-incidence thereto and with different types of grids or degrees'offocusing;

Fig. 7 is an exaggerated illustration of the shape .of the pattern ofphosphors upon adisplay screen with a target structure employing uniformspacing of apertures in the grid; and

Fig. 8 is a similarly exaggerated diagram illustrating the shape of theelectrode structure .for use with a screen. having a rectilinear patternof phosphors thereon.

One preferred form of the invention is embodied in a tube, basically ofconventional form, which is indicated at 1 of Fig. l. Such a tubecomprises the .usual evacuated envelope .3, which may be of al1 lglassconstruction or of metal and glass. lt has the usual viewing window 5 atits enlarged end and an electron gun 7 in the neck. Such a gun comprisesan electron emitting cathode 9, a control grid 11, a rst anode 13 and 4asecond anode 15. The tube, as shown for the purpose of illustrating oneform of deflection control, is provided with pairs of deflecting plates17 and 19 forl deflecting a beam ot cathode rays, produced by the gun,vertically .and 'horizontally respectively. lt is `to be understood,however, that, in the alternative, the more usual deflecting coils maybe used where desired. However, electrostatic deection is practical inthe tube of this type because of the relatively low initial velocity ofthe electron beam, and as will be shown hereinafter it possessesdefinite advantages with respect to color control.

Potentials for focusing and deflecting the beam are supplied by a radioreceiver symbolized by the block 21.

No specific description of this receiver is believed necessary, since itis essentially conventional.

The display surface or screen and electron lens `system indicatedgenerally by the reference character 23 will best be understood byreference to Fig. 2. Fig.- 2 shows the disposal of the color areas uponadisplay surface 25. Basically,as this ligure indicates, the three lphosphors which contribute luminescence in the primary colors of theadditive systems are disposed upon the base 25 in strips which extendcompletely acrossthe display area in one dimension. Strips 25,1 ,arecontinuous, 'consisting entirely of a phosphor luminescent in a singleprimary color. These strips are substantially uniform in Width and areparallel, spaced apart by a distance `Substantially equal to their ownwidth in this particular screen, although, as will be shown hereinafter,this is not a necessary condition. Intermediate strips 252 arediscontinuous, comprising alternate blocks, 252 and 25"z, of the tworemaining primary colors. The blocks 25'2, 252 as here shown are square,and the junctionsbetween the blocks are alined across the displaysurf-ace so that the blocks in any one row, transverse to the directionof the continuous strips, are all of one color.

There is no fixed rule as to which color of luminescence is provided bythe continuous strips and which by the discontinuous ones; in aheld-sequential system there is a slight advantage in making thecontinuous strips luminescent in green and the discontinuous onesalternately red and blue. This may also hold true for line-sequentialsystems. For certain dot-sequential system or for the constant luminancesystem, both of which may be considered as at least quasi-simultaneoussystems, there may be some advantage in making the continuous stripsluminescent in blue, but the advantage to be gained by any arrangementis not of suflicient importance to destroy the utility of a screen ofthis character for use with any presently known system, even if designedprimarily for a different one.

The embodiment of the invention shown in Fig. lmay be considered, forthe present purposes, as intended primarily for a field-sequentialsystem and therefore strip 2,51 has been indicated, by the letter G, asluminescent in green, withl the block 252 luminescent in blue and blocks25"'2 in red.

Spaced from the plane of the display surface 2S, by a distance short incomparison with the ytotal length of the path of the `electron beam, isa lens grid structure which is comprised of two sets of linearelectrodes, those in each set being parallel and substantially uniformlyspaced, although, as will hereinafter be shown, neither the width of thephosphor strip nor the spacing of the grid conductors is necessarilyexactly uniform. The non-uniformitics are, however, although important,very small indeed, The electrodes are conveniently tine wires, althoughthey may be narrow strips or tapes, mounted edge-on to `the beam path.In the particular tube shown the first set of these electrodes,designated alternately as 2,7 and 27", is mounted generally parallel tothe phosphor strips -on the display surface. Electrodes 27 are connectedto a common conductor 31; intermediate electrodes 27 are similarlyconnected to a common conductor 31. It may be noted that the mode ofsupport for .electrodes 27 and 27 is not important to this specificinvention and hence is not shown in detail. Various methods .ofconstructing such grids are shownin United States Letters Patents Nos.2,653,263 and 2,695,372 of Ernest O. lLawrence, or in United StatesPatent No. 2,721,288, granted'on October 1S. 1955, to .laines T. Vale.VLeads 8, connecting to conductors 31 and 31', are provided for applyingproper potentials to the electrodes of the structure, as by thehorizontal coloroscillator 33.

A `second substantially similar set of linear conductors 35 and 35",similar to conductors 27 and 27 is mounted as'closely as convenientlypossible to the rst set, between the latter and the electron gun. Leads39 conneet to these electrodes and are also brought out of the tube sothat the necessary potential relative to other elements can be appliedthereto by a vertical color control oscillator 37. Owing to thedifliculty of diagrammatic representation only two electrodes 35 and 35of this second set are shown in Fig. l.

An electron permeable electrode, substantially coextensive with thedisplay surface, is placed, with respect to the lens grid structure, sothat when proper relative potentials are applied to the elements of thesystem electrons from the beam passing through the lens grid will befocused substantially on the display surface. Such a afnemen film ismicroscopic in thickness and serves the triple purpose of Vestablishingthe lens forming the electric field with the lens grid structure,reecting luminescence from the screen outwardly through the window 5 andsuppressing secondary emission of electrons from the display surface.This film is not shown in Fig. 2 but is indicated by the referencecharacter referred in Fig. 1 to the surface of the base 25 and theconnection 41 for applying the focusing potential.

The width of the phosphor strips on the display surface should not begreater than the dimension of the minimum elemental areas or picturepoints which the tube, and the system in which it is employed, areintended to resolve. Fig. 2 shows the relative positions of the phospharstrips and the electrodes at the center of the screen. Viewed from thisaspect it will be seen that electrods 27 bisect the continuous strips251, while the electrodes 27' similarly bisect the discontinuous strips252. Similarly the transverse electrodes 35 bisect the blocks 252 in thetransverse direction while electrodes 35 bisect the remaining blocks252. In this portion of the structure a pencil of the electrons enteringa mesh of the lens grid formed by an adjacent pair of electrodes of eachof the two sets (if the potential of electrode 40 were the same as thaton the lens grid) would be -distributed over an area of the displaysurface of which one-half would be a portion of continuous strip 251 andthe other half which would be equally divided between portions of blocks252 25"2; the resulting emission would be, in the case here considered,one-half green and one-quarter each red and blue.

As has been shown in the prior patents previously referred to, if apotential is applied between the lens grid structure and the electrodes40 such that the latter is positive with respect to the lens grid, theelectrons entering the mesh can be brought into focus in the plane ofthe display surface and electrode 40. In this case an electron followinga path which is the average of all of the electron paths in the beamwould strike the display surface at the junction of the three differentcolor phosphors in the center of the quadrilateral area defined by themesh.

If, now, proper potential differences are applied between the electrodesof the two sets, the focal point will be shifted to a degree dependingupon the magnitude of the potentials applied. Undeected, theluminescence produced will be an unsaturated green. If the electrodes 35be made negative with respects to electrodes 35', the beam will bedeflected toward the latter and the resultant color will be yellow; if areverse potential is applied a blue-green will result. Making electrode27 negative with respect to electrode 27 results in mixed red and blue,or purple luminescence, whereas opposite deflection will give green.Electrodes 27 and 35 both made negative with respect to the otherelectrodes in their respective sets will give red luminescence whileelectrodes 27 and 35' nega tive will give blue. Increasing deliection inany of the directions mentioned will give increased saturation of thehues produced; up to the point where the entire focal spot falls on anarea emissive of a single color. If sinusoidal voltages are appliedbetween the electrodes in each set, and these voltages displaced inphase by 90 the focal spot will be spun in a circular path, the radiusof which will depend upon the amplitude of the voltages applied, and ifthe frequencies of the potentials causing the spinning are suflicientlyhigh so that the spot traverses all three colors within the period ofpersistance of vision, the psychological eect would be the same colorsas produced by the undeflected image, in this case an unsaturated green,three-quarters of the energy being white and the remaining one-quartergreen. Applying a fixed bias between conductor 27 and 27' the center ofthe circle about which the focal point is spun can be shifted so thatthe dwell of this spot on each of the three colors is equal, with aresultant pure white.

It will be seen, therefore, that each mesh of the lensgrid defines anarea of the display surfaces in which all electrons entering that meshmay be brought into focus and that this area is so divided that one-halfis luminescent in one primary color and one-quarter luminescent in eachof the two others. In the center of the screen the area thus defined isvery slightly larger in size than the lens grid mesh and lies directlybehind the latter as viewed from the electron gun.

Because of the angles formed by the beam with the axis of the tube asits deflection increases, the correspondence in size between the meshesand the corresponding subareas of the screen (which may be referred toas phosphor groups or color cells) is not exact nor do the displayscreen areas lie perpendicularly behind the meshes which converge theelectrons upon them except at the center of the screen. In spite ofthese facts the relative positions of any individual mesh and the centerof the area of the display screen controlled thereby may be computed.The velocities of the electrons in the beam are proportional to thesquare roots of the potentials through which they have fallen. In orderto scan any portion of the electron image the ratio betweenthetransverse and the longitudinal velocity of the electrons is known,this being the tangent of the angle of deflection. In the absence ofdeflecting potential between the electrodes of the grid itself, theelectron following the mean path of those entering a mesh-i. e., onepassing through the center of the mesh-will retain its same transversevelocity in its passage between the lensgrid and the display surface.The average longitudinal velocity of the electrons between the lens gridand the display surface is their velocity at the lens-grid plus one-halfof the difference between that and their final velocity. Thedisplacement of the point of impact of the mean-path electron from thebase of the perpendicular dropped from the center of the mesh to thedisplay surface will therefore be defined by an angle whose tangent isthe transverse velocity over the mean velocity between the lens grid andthe display surface; the distancev between the plane of the lens gridand the display surface being known, this defines the center of the areacontrolled by the individual mesh in question; the positions of thecenters being defined and being also arranged in a regular quadrilateralarray or repeating pattern, this establishes the areas themselves.

Stated somewhat more concretely, the velocity of electrons arriving atthe lens grid is determined solely by the difference of potentialbetween the latter and the cathode. The longitudinal component of thisvelocity is proportional to the cosine and the transverse componentproportional to the sine of the angle between the path of the electronand the perpendicular to the lens-grid at the lensgrid itself. Thevelocity of the mean path electron as it reaches the display screensurface will again depend wholly upon the potential difference betweenthe screen and the cathode, but the potential gradient between the lensgrid and the screen will add velocity to the longitudinal componentonly; the transverse component will be unaffected.

If a tube of the character here described is to be operated to reproducetelevision images in substantially natural or true colors, with equalcolor fidelity throughout the screen, the centers of the color cells orphosphor groups must be electro-optically alined with the grid mesheswhich control them, and the electron beam as it passes through anyindividual mesh must be converged to such an extent that it may be sodeflected as to fall on one phosphor only of the three comprising thegroup. These two requirements are interdependents. The degree offocusing desirable depends to some extent upon the system utilized fortransmitting the color information. Where a line-sequential system orfield-sequential system is used it may be desirable to make the degreeof focusing or convergence just sufficient to insure that the beam maybe confined to a single color phosphor when it is deected. Withdot-sequential or quasi-simultaneous systems it is usually preferable tomake the spot as small as possible,

but since the focusing varies with the degree of scanning 7 deflectionof the beam it may be advisable to choose a compromise degree offocusing which brings the spot to minimum size at some portions of thescreen other than the central area.

If any post deection focusing is to be used, however, the targetstructure must be designed so as to aline the grid apertures and theircorresponding color cells for the degree of focusing desired. ln orderto make such tubes commercially feasible the arrangements should be suchthat some degree of tolerance in spot size and focusing potentials isallowed for, even when it is desirable to use a focal spot of themaximum permissible size. What the factors are which control thepositioning of the phosphor cells and the apertures will be consideredin detail herein* after.

Since the principles involved apply generally to tubes using thepost-detiecton focusing principle, however, and since the principles aremore simply applied in connection with a tube wherein the lens-grid liesin a single plane, the factors involved will rst be considered inconnection with a tube utilizing a screen whereon the phosphors aredeposited in a pattern indicated generally in Fig.k3. Since the tube inwhich a screen of this character may be employed need differ from thatshown in Fig. l only in the omission of one of the two lensgrids and itsconnections, e. g., the grid comprising conductors 35 and 35 and itsconnections with the vertical color oscillator 37, no separate diagramof the tube is believed necessary.

As in the case of Fig. 2 the illustration is of only a small portion atthe center of the screen. In this case the phosphors are disposed instrips 45B, 45ex and 45B, indicating strips of red, green, and bluerespectively arranged in the order red, green, blue, green, red, etc.,two green strips being included for each one of the two other colors.The strips forming the screen in this case are parallel or very nearlyso, as shown the strips are all of equal width, but this is not anecessary feature. Linear conductors, for example, the wires 47 and 47extend across the screen in a direction generally parallel to thedirection of the strips, the wires 47 being centered in front of the redstrips 45a and the wires 47 similarly centered above the blue strips 45Bat the center of the screen depicted in Fig. 3. this geometricalpositioning no longer obtains, owing to the curvature of the electrontrajectories as described above and to be considered more fullyhereinafter. The apertures and the color cells are, however, effectivelyelectro-optically alined throughout the target.

The electrodes 47 correspond in connection to electrodes 27 of Fig. l,while electrodes 47 correspond to electrodes 27. The similarlydesignated electrodes are electrically connected and deflectingpotentials are applied from the horizontal color control oscillator 33,it being recognized that the electrodes may run either horizontally orvertically and that horizontal is in this case only specified as amatter of convenience, although there are certain advantages in havingthe color strips and electrodes vertical upon the screen and effectingthe color control by horizontal deflection, and in the discussion itwill be assumed that this is the case.

A consideration of the showing of Fig. 3 will make it at once evidentthat if the tube is to be able to display a substantially pure color thebeam must be converged so that its width is no greater than one-half thepitch of the grid electrodes 47, 47', even if at maximum it is deflectedso far as to center its spot of impact on the screen directly under thecolor-deocting electrodes. Under these conditions of minimum focusingthe width of a color cell or phosphor group under the control of any onepair of electrodes is approximately one and one-half times theelectrode-spacing or width of the aperture. Each red and each P lliecolor strip is shared by two adjacent color cells. As in apractical tubethe spacing of thegrid wires will be something of the order of 30 mils,each individual color strip will, in the pattern shown, be one-halfthis, or

At points more remote from the screen centery l5 mils wide. A one milerror in relative positions of the electrodes and their` correspondingphosphor strips therefore amounts to a 6% percent error in spacing. Forthis reason, even where relatively large spot size is desired because of.the system of color transmission used, it is desirable .to make thebeam converge to an extent greater than .the theoretical minimum so asto permit a reasonable degree of manufacturing tolerance in constructingthe apparatus and to allow for variation in sensitivity to colordeflection with varying angles of incidence at the grid.

Where the aperture is between linear conductors whose length is great incomparison with their separation the convergence of the beam can beexpressed by the equation V1 :cathodegrid voltage Vz=gridscreen voltageAy=convergence of beam yo=width of aperture between grid conductors=angle of incidence of beam at grid =oomponent of angle of incidencenormal to grid conductors The two components., a and ,3 of the angle ofincidence 0, respectively parallel and normal to the grid conductors,are related by the equation tan2 l9=tan2 +tan2 (2) It is convenient tocall the ratio since this quantity will appear repeatedly in thediscussion to follow. Using this notation, at the center of the screenwhere a=0, =0, Equation l reduces to Ay K n ya 1+\/1+A The width W ofthe spot is equal to 1.) M y 1+ ya y0 being a negative quantity. If W isnegative the spot is overfocused, the focal point being between the gridand screen, the beam diverging after it passes through focus. If

W should theoretically be zero, and the spot a geometrical line withoutwidth.

'In practice a focus as line as this is not obtainable for severalreasons. Aberrations are present in the electron lens, and the equationis strictly correct only for paraxial rays of the electron beam. Thepaths of the electrons within the beam are not truly parallel, althoughthey are very nearly so. Scattering of electrons takes place in themetal film 40 which covers the phosphor layer and within the phosphorlayer itself, and the light emitted by the phosphors is furtherscattered by the phosphor crystals.

In spite of these facts the equation gives a very fair guide to thedegree offocusing which can be realized. Curve 49 of Fig. 4 isagraphical representation of Equation 3, and curve Slof the same figureshows the size of spot obtained as measured on an actual tube, withvalues of K ranging from zero (no post-deflection focusing) to eight. lnthis curve the actual spacing between the wires of the grid was 231/3mils. The upper scale of the ligure is given in terms of the quantity K;in the lower scale the figures represent the relative voltages of screenand grid with respect to cathode, which is K l.

l't will be noted that the experimental curve lies for the most partalmost uniformly three mils above the theoretical curve, indicating thatthe various factors mentioned serve to increase the width of the spot byapproximately this amount irrespective of the amount focusing used. Suchwidening of the spot occurs even where no post-deflection focusing isemployed, and is in fact most pronounced at minimum focusing voltages,where the apparent width of the beam is greater than the apparent widthof the aperture. This would appear to indicate that the scattering ofthe electrons and the light are the factors which are most important incausing the increase in spot size. Nonetheless the experimental curvebrings out clearly the fact that the finest focus is obtained at thecenter of the screen when the factor K=3, as predicted by the equation.

It may be noted here that the equation given is that applicable tocylindrical lenses. lf a grid, still lying in a single plane, is tofocus the beam in both dimensions, Equation l must be modified bydividing the right hand quantity by 2. This comes about because thelines of force which tend to converge the electrons are divided betweenthose which cause convergence parallel to the a plane and those whichcause convergence parallel to the plane. With this modificationtheoretically perfect focusing is obtained when K=8 in Equation 3. Thisis true irrespective of whether the aperture be square,

p hexagonal (as shown in Figs. ll and l2 of United States Letters PatentNo. 2,692,532 referred to above) or circular. lf, however, one dimensionis greater than the the other the lens approaches the cylindrical form,becoming highly astigmatic, and there is no focusing potential whichwill make it of minimum size in both dimensions.

Aberrations become more apparent as the grid apertures depart from thecircular form, but the equation still offers a reasonable guide fordesign purposes. Thus with a grid having square apertures there is acertain amount of mutual shielding at the corners where theperpendicular conductors forming it join, which results in the fieldbeing weaker at the corners and stronger at the centers of the sides.The result is a pin cushion distortion, and since the intent is not toform a true image but merely to concentrate the beam, it may in someinstances be desirable to overfocus those portions of the beam enteringadjacent to the center of the sides of the apertures to such a degreethat the corners of the pin cushion, although possibly stillunder-converged, come in to an equal radius with the expanded sides.

Furthermore, particularly when wide angles of scanning deection areused, it may be desirable to underfocus the beam somewhat at the centerof the screen in order to cause less overfocusing at the edges, thesmallest focus being at some point between the center and the edges ofthe field. What constitutes the best focus is therefore dependent, insome degree at least, on conditions having no direct relation to thisinvention, except as they determine the post-deflection acceleration tobe applied between the grid and the screen.

Once the ratio of the voltage between cathode and grid to that betweenthe grid and screen has been determined, however, the necessaryrelationship between the phosphor groups and the apertures through whichelectrons reach those groups becomes fixed within a relatively narrowrange. lf the distances from the center of' beam defection to the gridand the screen are called Rq and Rs respectively, and the distances of aspecific aperture and the phosphor group whereon electrons through thataperture impinge from the screen center are termed Sq and Ss, withoutpost detiection focussing Rs T2? should equal Ss Sa The ratio of thespacing between the centers of any pair'l of apertures and theircorresponding phosphor groups would then be equal in all parts of thefield and measured in any direction; the positions of apertures andcorresponding phosphor groups would follow the similar triangle law,where Sq=Rq tan 0 and Ss=Rs tan 0.

As soon as post-defiection focusing is employed, however, the simple,similar-triangle relationship no longer follows. Neither, however, doesthe relationship hold which has sometimes been assumed by writers whohave treated post-deflection focusing, i. e., that the ratio betweengrid spacing and phosphor group spacing should be unity. The differencebetween these spacing-ratios is numerically small; in a practical tubewherein the maximum scanning-deflection angle 0 of the beam is 36 thedifference between the two ratios suggested by the prior art is only 5%,using grid to screen spacings (RS-Rg) approaching the maximum valuesthat have thus far been used in practice. Cumulatively, however, thisdifference in ratio amounts to several phosphor groups. As will next beshown, the proper value of spacing-ratio lies between unity and thesimilar-triangle value.

lf D is the distance between the screen and the grid, d is thedisplacement of the center of the phosphor group from the base of aperpendicular dropped from the center of the corresponding aperture,(the distance d bcing measured in the plane of the angle of scanningdeilection), the relationship given verbally in the broad description ofthe invention can be expressed by the equation or, using the samenotation as was used in discussing focussing,

= 2D tan 0 (5) 1+t/1+K S602 a) In the case of a screen such as is shownin Fig. 3, comprised of linear phosphor strips, each of a single colorphosphor, displacement of the point of impact in the direction parallelto the strips and to the general direction of the grid conductors has noeffect upon the color displayed. It is only the deflection traverse tothe strips which is important. The displacement in this direction can bederived from the Equation 4 or 5 and written:

In a typical tube in which a screen of the type illustrated in Fig. 3 isused, the aspect ratio of the screen will be, under present standards,4:3. With a 10%X14% screen, having a diagonal dimension of 18, theaverage width of the phosphor groups, measured from the center of a redstrip to the center of the nearest blue strip, will be about 30 mils,and the maximum angle of deflection, on the diagonal, may be taken as36. Under these circumstances the maximum value of the angle a, parallelto the grid wires will be approximately 231/2 while the maximumcomponent normal to the grid wires, will be 30.2 approximately. Thenumber of phosphor groups or color cells will be 479, formed between 480grid wires.

Curve 53 of Fig. 6 is a plot of the quantity dp against tan at the axisof the screen, where a=0. Curve 55 is a similar plot at the upper orlower edge of the screen where a=23.5. These curves are plotted assuminga value of voltage-ratio K=3, which gives the most concentrated focalspot at the center of the field, and with a value of D=450 mils. Forcomparison there is also shown a graph 56 of the displacement db inaccordance with the similar triangle law, where K=0; i. e., where nopost-deflection focusing is employed. The quantity 11 tan is, of course,directly proportional to the lateral dimension of the screen. It is alsoproportional to the number of phosphor groups, counting from the centerof the screen outward; exactly proportional if the strips comprising thegroups are all of the same width and very closely proportioned if thestrips are varied in width. Accordingly the lower scale of abscissas inFig. 6 is in terms of the phosphor group numbers, counted from thecenter of the screen. n

Since these groups are, onthe average, 30 mils wide, it will be seenthat with the assumed maximum values of the angie indicated on thegraphs by the points 57* and 59, the displacement d is equal to thecombined widths of several phosphor groups; 5.4 such groups on thehorizontal axis of the screen and 5% such groups at the upper and loweredges. The displacement also differs from the displacement requiredunder the similar triangle law by from 31/3 to 3.51 phosphor groups ataxis and corners of the screen respectively.

The relationships between the point of impact of the beam passingthrough a particular aperture and the width of the phosphor groups maybe more clearly seen from the diagram of Fig. 5, whereon is shown anelectron trajectory between grid and screen, using the same voltageratio K that was assumed in plotting the curves of Fig. 6 that have sofar been discussed. In Fig. 5 the broken line G indicates the plane ofthe grid, and the solid line S that of the screen. The solid line 63indicates the path of an electron passing through the center of anaperture at the edge of the screen (==approxi mately 30.2). The smallcircles 47, 47 indicate the conductors so numbered in Fig. 3. rl`he dashline 63 indicates the path which would be followed by the electron ifthere were no potential difference between the grid and the screen (K=0)and the broken line 65 is the perpendicular from the center of theaperture from which the displacement d is measured. The slope of line 66is inversely proportional to the average velocity of the electronsbetween grid and screen. The curved portion of the solid line 63indicates the parabolic trajectory of the electron between the grid andthe screen. The short dashes across the line S indicate the edges yofthe color cells. The drawing is approximately to scale and indicatesclearly how far the electrons would miss the color cells upon which theywere intended to impinge if the screen were so dimensioned that therespective color cells lay either directly behind the centers of theapertures or followed the similar-triangle law.

lt will be seen from Figs. 5 and 6 that the ratio of the spacingsbetween the centers of the phosphor groups and the centers of thecorresponding apertures should be always greater than unity and lessthan the ratio of the distances from the center of deflection of thebeam to the screen and the grid respectively if the phosphor groups andthe apertures are to be substantially electro-optically alined; i. e.,if a converging beam through the aperture is to fall entirely within thelimits of a single phosphor, emissive of the same color, when it isdirected to any part of the screen.

As should be apparent from the general discussion of focusingrequirements given above, such electro-optical alinement, although itmust be substantially correct, need not be mathematically exact. Thetolerance permitted depends both upon the screen pattern and the systemof transmission, as well as upon the maximum angle of deliectionemployed. It will be seen from the curves of Fig. 6 that the curvatureof the lines 53 and 55 is quite small. These curves can, therefore, beapproximated by singie straight lines in cases where a considerabletolerance can be permitted. Increasing the gridtoscreen distance Ddecreases the tolerance, as does the use of dotsequential systems oftransmission, where the beam is switched from color to color during thetransmission `of picture information, as contrasted with lineor tieldsevof 316% of the phosphor group width.

12 quential systems where the switching occurs during the blankingperiod.

The present tendency is toward shorter tubes using wider angles ofdeliection. As is indicated by curves 53 and 55 the rate of curvatureincreases with increasing deflection, and therefore approximations whichare per-k fectly valid with small maximum deflections become inadequatewith greater ones. With deilections of no greater than 30 andgrid-to-screen distances of approximately ten times the width of aphosphor group, the size of the screen being the same as that heretoforeconsidered, the widths of the phosphor groups and the spacing of thegrid wires can both be uniform without involving errors of more than 2mils at the corners of the screens and with errors of less than one milover the greater portion thereof if the proper average spacing-ratio ischosen. The errors which can be tolerated are, of course, dependent uponthe maximum dimensions of the luminous spot upon the screen as comparedto the dimensions of the sub-elemental areas of phosphor which itexcites.

The curves of Fig. 6 are drawn for both greater maximum angles ofscanning deilection and a greater gridto-scre'en spacing than aresatisfactory for use with a constant ratio of grid wire pitch tophosphor group Y width. Either the wire pitch or the phosphor groupspacing may be varied either continuously, in order to providesubstantially perfect correction, or in zones to apply corrections whichare completely adequate in a practical tube and which do not involveerrors greater than those inevitable in Acommercial manufacture. Atpresent the zone construction is preferred, since the changes in pitchas between adjacent apertures or phosphor groups are extremely minute-afraction of a millionth of an inch on the average-and it is much morepractical to make the corrections when they have become cumulatively ofappreciable value.

As the curves of Fig. 6 show, the 14% inch sc reen in the tube describedis only 324 mils wider than the corresponding grid on the central axisof the screen and only 310 mils wider at the top and bottom, adifference of only 14 mils as between center and edges. The totaldiffer.- ence in screen and grid dimensions is only 2%%. Thus, takingaverage values, if the widthof'the color cells is 30 mils, thecorresponding wire pitch is 29.325 mils, a difference of very slightlyover 2/3 mil.

For substantially exact compensation the ratio of the spacing should besuch that the phosphor groups are about 2.44 percent wider than the gridwire spacing` at the center and about 1% percent wider at the corners ofthe field.

Either curve 53 or 55 can be approximated by no more than three straightlines without involving an error of more than l mil at any part of thefield, a maximum If errors of as much as 10% can be tolerated, the samespacing ratio may be used at the center and edges of the field. Whereonly this first order correction is employed the correct spacing ratiomay be approximated by the use of straight grid wires and strictlyparallel and straight phosphor strips. The displacements necessary forsuch an approximation may be derived from a curve drawn midway betweencurves 53 and 55. Preferably, however, a second order correction isapplied. This involves making the ratio of phosphor width to grid pitchgreater at the center of the eld than it is at the edges, to conform tothe very slight barrel distortion of the rectilinear eld caused by thevariation in refraction of the beam with focusing angle. This barreleffect is only about 14 mils in 14 inches or in the neighborhood of1/10th of l per cent.

The first order correction of phosphor-width to gridwire-pitch ratio canbe made in two Ways; the Width of the phosphor groups may be variedwhile the grid pitch is kept constant throughout the width of the screenor the pitch of the grid wires may be varied while the width of thephosphor groups is maintained constant. Each of gramas;

these arrangements has certain advantages of its own. Because, asdisclosed in a 'copending United States patent application of thepresent inventor, Serial No. 399,753 the sensitivity to deflection ofthe spot varies with variation in the angle 0. In accordance with a lawhaving much the same form as that relating to the size of the focalspot, and because the variation in grid pitch (if this is the method ofcorrection employed) is in the right direction to compensate partiallyfor the increase in sensitivity, there is some advantage of using thismethod of correction. Copending United States patent application ofRobert Dressler, Serial No. 343,834, filed March 23, 1953, discloses amethod of positioning the grid wires at a high degree of exactitude.Using, preferably, the curve 55, indicating the relative displacement ofthe grid apertures and the phosphor groups, the curve may beapproximated by two or more straight lines; it is seldom necessary touse more than three. In accordance with the Dressler disclosure, thewires are positioned by bars of glass or like material which areaccurately notched and are mounted at the edges of the screen, one oneach side of the viewing area. The wires are stretched across these barsand are positioned by the notches therein. All of the notches can bemade simultaneously with an array of accurately spaced cutters mountedon a single shaft, and since the accuracy may be built into the cuttersthe actual formation of the notches becomes a simple and inexpensiveprocess. If, say, the 250 wires at the center of the screen are disposedwith uniform pitch, the next 75 wires on each side of the central groupare disposed with a slightly increased but uniform pitch and a finalgroup, with a pitch again slightly increased extends to the edge of thescreen, the error in the tube used for illustration, may be made lessthan one mil at any position along the upper and lower edges of thescreen.

Alternatively, the screen may be laid out to a very large scale with thewidths of the phosphor groups Varied in the inverse manner; the width ofthe phosphor groups will be greatest in the central portion of thescreen with zones of successively decreasing width as the edges of thescreen are approached. The design thus constructed may be then reducedphotographically to proper size and silk screen stencils or otherprinting devices made therefrom. If the particular service in which thetube is to be employed will permit a compromise in which only the firstorder correction is necessary it is obvious that a curve intermediatecurve S3 and 55 can be constructed in the same manner to determine gridpitch or phosphor group spacing as the case may be.

It is preferable, however, that the second order correction be made. Thesecond order correction can also be applied in two different ways,irrespective of the method employed in the rst order of correction. Oneway which has proved very satisfactory in practice is to make a gelatinprint of the screen pattern, mount this print in a frame, the sides ofwhich may be bowed outwardly, and stretch the frame and the gelatinprint by the necessary 14 mils (in the present case) to accomplish thesecond order correction. The gelatin print can then be rephotographed,while stretched, and the resulting negative used as a master from whichany desired number of duplicates may be formed.

In the alternative, the second order correction can be applied to thegrid, whether or not the first order correction was so applied.Copending United States patent application of Howard R. Patterson,Serial No. 364,778,

tiled June 29, 1953, discloses the use of damp rods of undulatory formwhich may be used to prevent vibration of the grid wires under theelectro-static forces set up by the color-switching process. These damprods may be preformed, as shown in the application mentioned. Theundulations in the damp rods, passing under and over alternate gridwires, serve to position these wires laterally and by forming the damprods with the correct pitches for Various zones, longitudinally of theWires, the grid as a whole can be pulled into a slightly pin-cushionform which will give the desired correction.

The two general alternatives are illustrated in Figs. 7 and 8respectively. Because it would be impossible, in a patent drawing, toshow either the number of grid conductors or the number of phosphorstrips actually used, both the relative widths of the phosphor groupsand the magnitude of the correction is shown as applied in three zones,one central and two with equal and opposite cor- Vrections at the sidesof the screen. The second order correction is shown as applied only atthe two outer zones. Successive color cycles, red, green, blue green,are `shown cross-hatched in opposite directions and the junctionsbetween the individual phosphors are indicated by dotted lines. In Fig.7 the grid electrodes are shown as straight and uniformly spaced and thecorrection is applied to the phosphors; in Fig. 8 the reverse is true.The reference characters on the few parts shown correspond to those usedin Fig. 3.

It will be recognized that the particular methods of varying the ratiobetween aperture spacing and phosphor group spacing that have beenmentioned are only a few of those which may be employed to securesubstantially the same results.

The factors leading to the grid-screen relationshipl here stated havebeen 4given thus at length in connection with the single-grid structuretypiiied by Fig. 3 because of their relative simplicity and becauseprecisely the same principles are involved as in the case of adouble-grid structure of the type illustrated in Figs. l and 2. Whateverthe structure or whatever the degree of focusing employed, thedisplacement d is proportional to the component of velocity imparted tothe beam by the scanning deilection, measured in the direction of thatcomponent of velocity, multiplied by the electron transit time betweenthe plane of the grid and the plane of the screen. In the case where twogrids are used the focusing effect can be computed in accordance withthe same general method employed in the case of a single grid, but thiscan only be done with a degree of accuracy which approximates thatobtainable in a single grid if the two grids either lie so closetogether that they may be treated as if they lay in a single plane andEquation l modied by multiplying the right-hand terrn by 1/2 may beapplied to give nearly correct results or if they are separated by adistance which is large in comparison to the separation of the gridwires, so that each grid, when viewed from the other, can be treated asif it were an equi-potential surface. At positions between the twomentioned the fields between the wires of the two grids are sutilcientlywarped to introduce inaccuracies which are greater than those obtainedwith the focusing formula given in Equation 1, supra.

With these reservations, however, the approximate convergence of thebeam by the respective grids can be expressed by the equations:

For the rst grid:

tan

As used in these equations J and K are respectively the ratio of theintergrid voltage kand of the second-grid-toscreen voltage to thevoltage between the cathode and the irst grid; C is the distance betweengrids and D' is the distance from the `second grid to the screen. Theangle a, measured parallel to the electrodes of the first grid is ofcourse transverse to those of the second.

As is the case of the single dimension focusing, the minimum-size spotis obtained whenthe quantity on the left of each of these equationsbecomes -lg when C becomes small as compared to the pitch of the gridelectrodes the assumptionof uniform eld between the two grids, on whichthe focusing equations are based, is no longer valid. As the two gridsapproach the same plane, however, J approaches zero and K approaches 8for a spot of minimum size at the center of the field. The displacementd under these circumstances is represented by curve 67 of Fig. 6.

ln the form of the invention illustrated in Figs. l and 2 it is usuallydesirable to separate the grids by a greater distance than would warranttheir consideration as being in the same plane, since the latterarrangement accentuates thepinucnshion distortion of the spot. Itis alsodesirable, however, to space theA second grid from the screen by adistance fairly large in comparison to the pitch of the grid wires.

1n one such tube the spacing D' from second grid to screen is 360 milsand the inter-grid spacing 80 mils, the pitch of both grids beingsubstantially equal and approximately 30 mils. With the proper focusingvoltages this gives a substantially square spot.

Where the grids are so closely spaced as to give pin'- cushiondistortion it may be reduced in eifect by overfocusing, increasing Jslightly and K' materially.

As in the case of the single grid the convergence of the beam increaseswith the angle 0. There may therefore be a definite advantage inunder-focusing at the center of the screen to avoid excessiveover-focusing at the edges thereof, in order to obtain the best averagefocus.

The pattern of Fig. 2 possesses the unusual character- 1 istie that itpermits the display of saturated colors even where the dimensions of thespot are equal to those of the aperture, provided the deflection of thebeam is correct to centerthespot on the proper color phosphor. As aresult it permits of a large latitude in the choice of the degree ofpost-deection focusing to be used,-par ticularly when employed withmulti-gun tubes where the factor of rcolor-deiiection sensitivity doesnot enter; In

such multi-gun tubes it may be desirable to converge the beam onlyenough to allow `for a degree of manufacturing tolerance, perhaps 10% ofthe aperture width at the center of the screen. For tubes using gridcolor deflection acceptable results with either sequential or NTSCsystems of color transmission are obtainable when the ratio between thevoltage of thecathode and the mean of the two grids to that betweencathode and screen is l :4, which leads to spot displacements andaperture to phosphor-group spacing ratiosy Vsubstantially as given bythe single-grid formula above, provided the scanning deiiection anglesemployed are not too large.

In general, however, it is preferred to employ the more accurateformulas:

, more or less widely than those of the other.

As far as the amount of the displacement produced is concerned the onlyeffect of either grid upon the other is its effect upon the component ofthe velocity normal to the screen. The wires of either grid may beseparated In the case of the screen shown in Fig. 3, the continuousstrips may run generally parallel to the wires of either grid, Thebarrel distortion of the field will occur no matter whether it be therst or the second grid which causes it, although the amount of thisdistortion will differ as between the two grids. Since, as will be seenfrom curve 67 of Fig. 6, where K=8, the total displacement as betweencolor cells and grid apertures is less when bidimensional focusing isused, because of the greater average velocity of electrons between thegrids and the screen, the amount of barrel distortion will also besmaller, but it will still exist. It follows that the positions of theblocks transverse to the continuous strips will also be subject to thebarrel distortion. l

It should be noted that although applying a pin-cushion correction tothe grid causes electrons falling through it to be properlyelectro-optically alined with the corresponding phosphor group, it doesnot correct the small amount of barrel distortion which exists in thepicture field. Even though the grid may be shaped to correct thepin-cushion distortion as far as the color displayed is concerned, thescanning deection will ordinarily be rectilinear and will not follow theform of the grid and hence the barrel distortion will still appear onthe screen. As has been noted, however, the total amount of thisdistortion is very small and cannot ordinarily be noticed. Usually it isless than the distortion inherent in the seanning waveforms developed inthe television receiver.

Any of the methods of compensation that have been discussed inconnection with a single-grid type of target may be utilized withdouble-grid targets, as can any combination ofY these methods ofcorrection. It therefore appears unnecessary to reconsider thesemethods.

One point which should be noted, however, -is that if electricaldeflection is used, ias is shown in Fig. l, the centers of detiect-ionwill be different with respect to the two dimensions of deflection.Where electro-magnetic deflection is used the -coils -in the deflectingyoke are normally -in the .same plane and the center of deflection willbe the same for both directions of scanning deflection.

The degree of focusing desirable with double-grid tubes using a phosphorpattern of the type illustra-ted in Fig. 2 depends in some degree on thesystem by which color information is transmitted. 'Single-gun' tubesus-ing this type of pattern may be used with the NTSC system withoutbreaking up the substantially :continuously Atransmit-V ted -colorinformation into dot-sequential form. Using the .pattern of Fig. 2 .thetransition from any color to any other .can be made direc-tly as it isunnecessary 'to traverse the green strip, for example, `in passing fromred to blue and the electrodes .can be so biased that when no colordeflection is applied Ithe resultant light is a pure white. The colordisplayed then depends on the direction of deflection and the saturationon its amplitude.

Under these circumstances a somewhat more pleasing effect is producedwith a diffuse spo-t than with a sharply Having thus'set forth theinvention, what is claimed is as follows:

f 1. In a cathode-ray tulbe for displaying television images in colorincluding means for directing ya iiow of electrons against a target areaacross which they are adapted to be defiected` in two dimensions from -acenter of defiection to trace a raster defining a field of view; Iatarget insaid area comprising a display screen including a base,phosphors emissive on electron impact of light of different componen-tcolors additive to produce white light disposed over substantially 4theentire area of said base in a repeating pattern composed of groups ofall of said phosphors, the area covered by each group being in at leastone dimension of the order of magnitude of one elemental area of thetelevision 4image to be reproduced and an Aelectron permeable conductinglayer covering said phosphors; an electrode structure mounted adjacen-tand substantially parallel to said screen, 'and terminals for applyingdifferent electrical potentials to vsaid conducting layer and saidelectrode structure, respectively, said electrode structure having:apertures defining the pupils of a multiplicity of electron lensesthrough which said electron flow can be focused on corresponding lgroupsof phosphors, the centers of said groups of phosphors beingsubstantially uniformly spaced and the spacing of said apertures beingnon-uniform and greater near the edges of said target/than at the centerthereof.

2'. In a cathode-ray tube for displaying television images in colorincluding means for directing a flow of electrons against a target areaacross which they are adapted to'be deflected in two dimensions from acenter of deflection to trace la raster defining |a field of view; atarget in said area comprising a display screen including a base,

phosphors emissive on electron impact of light of different componentcolors additive to produce white light disposed over substantially theentire area of said 'base in a repeating 'pattern of substantiallyparallel strips of the respective phosphors so arranged that any threesuccessive strips comprises a group including all of said phosphors thewidth of which is of the order of magnitude of one elemental area of thetelevision images to lbe re-.

produced, and an electron permeable conducting layer deposited over saidphosphors; a grid of linear conduc.

tors mounted adjacent and substantially parallel yto said screen withthe conductors .thereof crossing said screen in the same direction assaid phosphorstrips and defining therebetween a multiplicity ofapertures through which 4said electron flow may be directed tocorresponding single groups of said strips, the ratio of the widths ofthe groups of three strips to the widths of the corre sponding aperturesbeing greater at the longitudinal centers'of the groups of strips thanlat the ends thereof.

3. The invention as defined in claim 2 wherein the centers of saidlapertures are more widely spaced at the ends of said linear conductorsthan .at their centers.

4. The invention as defined in claim 2 wherein each adjacent pair ofsaid linear conductors is uniformly spaced throughout its length andsaid groups of phosphor strips are wider attheir centers than at theends thereof.

5. 'In a cathode-ray tube for displaying television images incolor-including means for directing a ow of electrons against a targetarea across which lthey are adapted to be defiected in two dimensionsfrom a center of deflection to trace a raster defining a field of view;a target in said area comprising a display screen including a base,phosphors emissive on electron impact of light of different componentcolors additive to produce white light disposed over substantially theentire area of said base in a repeating pattern composed of groups ofall of said phosphors, the area covered by each group vbeing in at leastone dimension of the order -of magnitude of one elemental area of thetelevision image to be reproduced and an electron permeabley conductinglayer covering said phosphors; an electrode structure mounted adjacentand substantially parallel to said screen, and terminals for applyingdifferent electrical potentials to said conducting' constant, and thera-tio of aperture spacing to group spac-v ing increasing from yzone toZone -outwardly from the center of said target.

6. In a cathode-ray tube for displaying television im-f ages in colorincluding means for directing a flow of elec trons against a target areaacross which they are adapted y to be deflected in two dimensions from acenter of deflection to trace a raster defining a field of view, adisplay screen disposed in said target area comprising a base,

phosphors emissive on electron impact of light of dif' ferent componentcolors additive to produce. white light disposed on said base, and anelectron permeable conducting layer covering said phosphors, saidphosphors being disposed over substantially the entirearea` of said basein a repeating pattern composed of groups of all of said phosphors, thearea covered by each group being, in at least one dimension, of theorder of magnitude of one elemental area of the television image to be rreproduced, an electrode structure mounted adjacentand substantiallyparallel to said screen, and terminals 'for applying differentelectrical potentials to said electrode structure and said conductinglayer respectively, said electrode structure being apertured to definethe pupils of a l multiplicity of electron lenses through each of whichelectrons of said iiow can be focused on a single one of said groups ofphosphors, the ratio of'the spacings between the centers of saidapertures and the centers of the respective groups of phosphors on whichelectrons of said iiow entering said apertures impinge being less thanunity and greater than the ratio of the distance from said:

center of defiection to said electrode structure to the distance fromsaid center of deliection to said screen.

7. In a cathode-ray tube for displaying television images in colorincluding means for directing a flow of electrons against a target areaacross which they Vare adapted to be deflected in two dimensions from acenter of deflection to trace a raster defining a eld of View, aidisplay screen disposed in said target area comprising a v base,phosphors emissive on electron impact of light of different componentcolors additive to produce white light disposed on said base, and anelectron permeable conducting layer covering said phosphors, saidphosphors `being disposed over substantially the entire area of saidbase in a repeating pattern composed of. groups of all of saidphosphors, the area covered by-each :group bev ing in at least onedimension of the order of magnitude j of one elemental area of thetelevision image to be j reproduced, electrode structure mountedadjacent and substantially parallel to said screen and terminals forap-V v plying different electrical potentials to said electrodestructure and said conducting layer respectively, said electrodestructure being apertured to define the pupils of a multiplicity ofelectron lenses through each of ywhich electrons of said ow can befocused on a single one of said groups of phosphors, the ratio of thespacings between the centers of said apertures and the centers of therespective groups of phosphors on which electrons of said flow enteringsaid apertures impinge being less than unity and greater than the ratioyof the distance from said center of deflection to said grid to thedistance from said center of deliection to said screen, said ratio ofspacings being greater adjacent to the edges of said screen than at thecenter thereof.

8. In a cathode-ray tube for displaying television images in color,including an electron gun for directing a beam of cathode rays against atarget area across which yit is adapted to be deflected in twodimensions from aJ including phosphors emissive of all of said componentcolors being of the order-'of magnitude of one elemental area of thetelevision images to be reproduced, a grid of elongated linearelectrodes mounted in a plane substantially'parallel to said screen andadjacent thereto, and com prising two interleaved and mutually insulatedsets, terminals for applying different electrical potentials to theelectrodesy of said two sets and to said conductingV layer,

theV ratio of the spacings between thel electrodes of said j two setstothe spacings of the centers of said. groups being less than unity andgreater than the ratio of the distance between the center of deflectionof said beam and said grid to the distance from said center ofA delico.tion to said screen.

9., In a. cathode-ray tube for displaying televisionA irn.V

agesiin; color, including an electron gunfor-directing a beam of cathoderays against a target area across which it is adapted to be deflected intwo dimensions from a center` of deflection, to trace a raster denin'g afieldl of View, a display screen disposed in said targetxarea comprisingabase, strips of phosphors emissive on electron impact of light ofdifferent component colors additive to produce white deposited on saidbase in a cyclic order, and an electron permeable'conducting layerdeposited over said phosphor strips, the width of each group of stripsincluding phosphors emissive of all of said component colors being ofthe order of magnitude of one elemental area of the television images tobe reproduced, and each group of strips being wider at the center thanat'the ends thereof, a grid of elongated linear electrodes mounted in aplane substantially parallel to said screenV and adjacent thereto, andterminals for applying different electrical potentials to saidconducting layer and said grid, the ratio of the spacing of theelectrodes comprising said grid to the spacing between the centers ofsaid groups being less'than unity and greater than the ratio of thedistance from the center of deection of said beam to the plane of saidgrid to the distance from said center of deection to said screen.

10. In a cathode-ray tube for displaying television imagesin colorincluding an electron gun for directing a beam of cathode-rays against atarget area across which said beam is adapted to be deected in twodimensions from a center of deilection to vtrace a raster defining afield of view, a substantially plane display screen disposed in saidarea, said screen comprising a base, strips of phosphors emissive onelectron impact of light of different component colors additive toproduce white light disposed on said base in a cyclic order and anelectron.- permeable conducting layervdeposited over said phosphorvstrips, the width of each group of strips including phosphors emissiveof all of saidY component colors being of the order of magnitude of oneelemental area of the television images to be reproduced, a grid ofelongated linearl electrodes mounted adjacent to said screen in a planesubstantially parallel to the plane thereof, said electrodes beingoriented with their length in directions generally parallel to thelength of said strips, and terminals for applying different potentialsto said conducting layer and to said grid, the ratio of the spacing ofadjacent electrodes of said grid to the spacing of the centers 4of theadjacent groups of said strips nearest said adjacent electrodes beinggreater for pairs of adjacent electrodes at the edges of the screen towhich said strips are parallel'than for pairs of electrodes passing overthe center of the screen.

11. VIn a cathode-ray tube fordisplaying television images in colorincluding an electron gun for directing a beam of cathode raysl againsta target areaacrosgsgwhieh said beam is adapted to be detiected in twodimensions from a center of deflection to trace a raster defining a eldof view, a substantially plane display screen disposed-fili` said area,said screen comprising a base, strips of phonphors emissive-on electronimpact of; light of. dierent component colors additive to produce` whitelight vdisposed on said base in a cyclic order and anelectron-permeable, conducting layer deposited over said phosphorstrips,xthe width of each group of strips including phosphors emissiveof all of said component colors beingv ofY the order cimesnitude ofoneelemental area of the television imagestoby reproduced, a grid ofelongated linear electrodesJ mountedadjacent to said screen in a planesubstantially parallel t0 the plane thereof, andterminals for applyingdiierent pov; tentials tosaid conducting layer and to said grid, saidelec.- trodes being oriented in directions generally.y parallel to thelength of said strips, the spacing of said electrodes being uniform atthe midpointof the length of said elec! trodes and the spacing of thecenters* of. said groups of' strips being uniform at the midpoint of thelengthof said strips, the ratio of said electrode spacings to saidgroupspacings being less thanunity and greater. than theratio of the distancebetween said center of deflection and theplane of said grid to thedistance between said center of.

deflection and the plane of said screen. Y

l2. In a cathode-.ray tube for displaying television im.- ages in colorincluding an electron gunforv directing a beam of cathode rays against atarget area across, which said beam is adapted to be deflected in twodimensions from a center of deection to trace araster deninga leld ofview, a substantially plane display screen disposed in said area, saidscreen comprising a base, stripsof phosphors emissive on electron impactof lightof different component colors additive to produce white lightdisposed on said base in a cyclic order andan electron-permeableconducting layer. depositedv over said phosphor strips, the width ofeach group of strips including phosphors emis.v sive of all lof saidcomponent colors being of the order of magnitude ofl one elemental areaofthe television imf ages to be reproduced, allof saidV groups havingthe same.

f width, al grid of uniformly spaced elongated linear-electrodes mountedadjacent to said screen in a plane substanf tially parallel to the planeof said screenand lterminals for applying different potentials to saidyconducting layer and to said grid, the ratio of the spacing of,theelectrodes4 ofi said. grid to the width of said groups being'lessthan unity and greater than the ratio of the distance between saidcenterfof deflection and the plane of said gridto the distance betweensaid `center .of deiiection and" the plane of said' screen.

13. In. a cathode-ray tubeL for displaying television i111-,v ages incolor including means for directing a owofekC- trons against a targetarea across which they are adaptedv to be deflected in two dimensionsfrom a center'of' der ilectionv totrace a raster defining a ieldvofview, `a display screen .disposed in 'said target area comprising a bumphosphors emissive on` electron. impact of lighty of` differv entcomponent colors additive to produce white light dis: posed onsaid base,and an electron permeable conducting layer covering said phosphors, saidphosphors being: disposed over substantiallyvthe entire areak of saidbase-in n repeating. pattern composed of groups of all of said phosfphors, the dimensions' of all of said groups being-equalv ,and of theorder of'rnagnitude of one elemental area, of

the image to be reproduced, an electrode structure mount ed adjacent'andsubstantially parallel to said screen, and' salcl flow entering saidapertures impinge being uniform throughout said target area and lessthan unity and greater than the ratio of the distance from said centerof deflection to said electrode structure to the distance from saidcenter of deection to said screen.

14. In a cathode-ray tube for displaying television images in color,including an electron gun for directing a beam of cathode rays against atarget area across which it is adapted to be deected in two dimensionsfrom a center of deection to trace a raster dening a field of View, adisplay screen disposed in said target area comprising a base, andstrips of phosphors emissive on electron impact of light of diierentcomponent colors additive to produce white deposited on said base in acyclic order, the width of each group of strips including phosphorsemissive of all of said component colors being of the order of magnitudeof one elemental area of the television images to be reproduced, anelectron permeable conducting layer deposited over said phosphor strips,a grid of elongated linear electrodes mounted in a plane substantiallyparallel to said screen and adjacent thereto, and terminals for applyingdiierent electrical potentials to said conducting layer and said grid,the ratio of the spacing of the electrodes comprising said grid to thespacing between the centers of said groups being less than unity andgreater than the ratio of the distance from the center of deilection ofsaid beam to the plane of said grid to the distance from said center ofdeflection to said screen and said ratio of spacing being greater at theportions of the screen at the ends of said phosphor strips than at theportions of said screen dened by the centers or said strips.

l5. In a cathode-ray tube for displaying television images in color,including means for directing a ow of electrons against a target areaacross which they are adapted to be deflected in two dimensions from acenter of deection to trace a raster defining a substantiallyrectangular field of view, a display screen disposed in said target areacomprising a base, phosphors emissive on electron impact of light ofdifferent component colors additive to produce white light disposed onsaid base in a repeating pattern composed of groups of all of saidphosphors, the dimensions of each of said groups being in one dimensionat least of the order of magnitude of one elemental area of theltelevision image to be reproduced and said pattern coveringsubstantially the entire area of said eld of View, an electrodestructure mounted adjacent to said base and symmetrically disposed withrespect thereto and equidistant from the corners of said raster andhaving apertures therein defining the pupils of a multiplicity ofelectron lenses through each of which said electron ow can be focused ona single one of said groups of phosphors, at least one electrodepermeable to electrons between the phosphors on said base and saidelectrode structure, and connections for applying a diierence ofpotential between said electrode structure and said electron permeableelectrode, the ratio of the spacings between the centers of saidapertures and the centers of the respective groups of phosphors on whichelectrons of said flow entering said apertures impinge being less thanunity and greater than the ratio of the distance from said center ofdeflection to said electrode structure to the distance from said centerof deection to said screen.

References Cited inthe le of this patent UNITED STATES PATENTS 2,532,511Okolicsanyi Dec. 5, 1950 2,631,259 Nicoll Mar. 10, 1953 FOREIGN PATENTS866,065 France June 16, 1941

